1. Field of the Invention
The present invention is directed in general to wireless communication systems. In one aspect, the present invention relates to a method and system for improving the performance of wireless transceivers by providing an improved detector for multiple-antenna systems.
2. Description of the Related Art
Modern communication systems support wireless and wire-lined communications between a wide variety of wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth (BT), advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS) and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device (such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc.) communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over the tuned channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network, via the Internet, and/or via some other wide area network.
Wireless communication devices typically communicate with one another using a radio transceiver (i.e., receiver and transmitter) that may be incorporated in, or coupled to, the wireless communication device. The transmitter typically includes a data modulation stage, one or more intermediate frequency stages and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
The receiver is typically coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
In wireless communication systems utilizing the various 802.11 standards, the allowable bandwidth is set be standard-setting associations and governmental agencies. To achieve higher data throughput, many later generation wireless systems, such as those based on the 802.11n standard use Multiple Input Multiple Output (MIMO) antenna systems. MIMO systems use multiple transmit antennas to transmit multiple data streams in the same frequency spectrum and take advantage of multipath channels with a plurality of receive antennas being used to recover the information transmitted over the various data streams. Thus in a MIMO system, information is transmitted and received simultaneously using multiple transmit and receive antennas. In such a system, each pair of transmit and receive antennas defines a signal path from the transmitter to the receiver.
MIMO technology has been adopted by the Institute for Electrical and Electronic Engineers (IEEE) for the next generation wireless local area network (WLAN) to provide a throughput of at least one hundred Mbps. Transmission protocols and standards for such a high throughput (WLAN) are embodied in a standard referred to as 802.11n. Since 802.11n is a MIMO extension of current WLAN standards, such as 802.11a and 802.11g, 802.11n will also be based on the transmission scheme referred to as orthogonal frequency division multiplexing (OFDM).
A MIMO system can provide two types of gain: (1) diversity gain, and (2) spatial multiplexing gain. Diversity gain is realized when signals carrying the first information are sent via different paths. This multipath transmission increases the robustness of transmission or the reliability of reception. Spatial multiplexing gain is realized when signals carrying independent information are sent in parallel via different paths. This increases the length throughput or the data rate of the wireless communication system.
In MIMO systems, there is a need to obtain an estimate of the transmitted signal with a high degree of accuracy. However, there is an inherent tradeoff between maximum accuracy and the speed of processing the signal. The optimum detector is a maximum-likelihood detector. Given the received symbol vector y, the maximum-likelihood detector searches over all possible transmitted signals/vectors xj for the transmit vector that maximizes the conditional probability Pr{xj/y}, thereby minimizing the probability of decoding error at the receiver. Since communication systems will employ some form of coding, the output of the maximum-likelihood detector should be a measure of reliability of each transmitted bit. These reliabilities are also known as soft decisions. However, the maximum-likelihood detector involves searching over all the possible combinations of transmit symbols. For a system with multiple transmit antennas, the complexity grows exponentially with the number of transmit antennas. Fortunately, when the MIMO channels are well-conditioned, suboptimal equalization-based detectors are likely to be as good as the optimum maximum-likelihood detector. In other words, the highly complex maximum-likelihood detector is only necessary when the MIMO channels are ill-conditioned. For MIMO channels that are neither well-conditioned nor ill-conditioned, a reduced-complexity maximum-likelihood detector can be used. A reduced complexity soft-output maximum likelihood detector is described in U.S. patent application Ser. No. 11/027,106, filed on Dec. 31, 2004, by inventor Min Chuin Hoo, entitled “Reduced Complexity Detector for Multiple-Antenna Systems,” which by this reference is incorporated herein for all purposes.
In view of the foregoing, it is apparent that there is a need for an adaptive detector that exploits the fact that detectors of different complexities work best in different conditions. The purpose of the adaptive detector is to adapt the choice of the appropriate detector according to some signal quality measure and still give comparable performance to the optimum maximum-likelihood detector.